A dual-mode or multi-mode wireless transmit/receive unit has dual or multiple radio transceivers, each designed to communicate on a particular radio access technology (RAT), such as 3rd Generation Partnership Project (3GPP) and non-3GPP systems. The handover process between 3GPP and non-3GPP systems may be slow due to the nature of the system configurations and operations. One problem occurs when a WTRU moves from one system to another as the WTRU is required to register and authenticate in the other system. A similar problem exists for session initiation protocol (SIP)-based Session Continuity processes between 3GPP and non-3GPP systems. When moving from one system to the other, the WTRU is required to register and authenticate in the other system before registering with internet protocol (IP) multimedia subsystem (IMS).
Another problem may occur due to the 3GPP prohibition against simultaneous radio transceiver operation. A single WTRU cannot have a 3GPP radio transceiver and a non-3GPP radio transceiver active at the same time. In such cases, dual-mode or multi-mode radio transceivers need sophisticated control of the radio switching.